Coating compositions for precoated steel sheets are required to have various properties, such as high hardness, high workability, stain resistance, chemical resistance, water-proofness and distinctness of image. Of these, the hardness and the workability are important characteristics, which, however, are contradictory to each other but the levels of which are required to be high. The workability as referred to herein indicates that, when flat, coated metal sheets are worked into various shaped structures through bending, cutting or drawing, the paint films are damaged little. Therefore, it shall be said that the larger the degrees of elongation and flexibility of paint films themselves, the better the workability of the paint films. On the other hand, it is desirable that the hardness of paint films is higher in order to make them have higher durability, while paint films having higher hardness tend to have lower workability.
Regarding coating compositions for precoated steel sheets, polyester resins and epoxy resins have heretofore been used essentially in undercoating compositions for outside faces and in coating compositions for back sides, while polyester, acrylic or polyvinyl coating compositions in overcoating compositions for outside faces. Polyester resin coating compositions comprising a combination of polyester resins (main ingredients) and melamine compounds (curing agents) are the most typical as overcoating compositions for outside faces, in which the polyester resins give the workability and the melamine compounds give the hardness. However, their properties are not always satisfactory when they are used in precoated steel sheets which are required to have the highest level properties with severity, especially in those for household electric appliances, and therefore, the improvement in the properties of such overcoating compositions for outside faces is required.
Given the situation, various proposals such as those mentioned below have heretofore been made.
Japanese Patent Application Laid-Open No. 62-236817 discloses a resin composition for coating, which comprises a polyester polyol containing, as the glycol component, an alicyclic glycol such as 1,4-cyclohexane-dimethanol, and a blocked prepolymer having NCO at its terminal.
Japanese Patent Application Laid-Open No. 63-108081 discloses a coating composition comprising (i) a polyester polyol, in which the acid component comprises an aromatic dicarboxylic acid including terephthalic acid, and an aliphatic carboxylic acid, and the polyol component comprises ethylene glycol and 1,4-cyclohexane-dimethanol, and (ii) a curing agent.
Japanese Patent Application Laid-Open No. 59-11370 discloses a coating resin composition comprising (i) a combination of two different linear polyesters to be prepared from acid components of terephthalic acid, isophthalic acid and an aliphatic dicarboxylic acid, and glycol components of ethylene glycol and neopentyl glycol, and (ii) a methoxymelamine compound and a butoxymelamine compound as combined with (i).
Japanese Patent Application Laid-Open No. 62-195057 discloses a coating composition for precoated steel sheets, which comprises (i) an urethane polymer resulting from the introduction of an urethane bond into a polycondensate of a hydroxyl-having copolyester resin with a polyisocyanate compound, (ii) a hydroxyl-containing copolyester resin, and (iii) an alkyl-etherified melamine resin.
Japanese Patent Application Laid-Open No. 2-209979 discloses a coating composition for precoated steel sheets, which comprises (i) a saturated polyester resin in which the acid component consists essentially of an aromatic dicarboxylic acid (or its ester) and the glycol component comprises an alkylene oxide low-adduct or high-adduct to bisphenol A, and (ii) an alkyl-etherified formaldehyde resin.
Japanese Patent Publication No. 56-18032 discloses a technique of dispersing rubber particles in an acrylic resin composition to thereby reduce the work strain of the film made from the composition.
Japanese PCT Application Laid-Open No. 3-500544 discloses a technique relating to the coating film to be formed by reacting an acrylic polymer or polyester polymer having a mesogen group as bonded thereto through covalent bonding and having a low glass transition point with a curing agent such as an amino resin.
Japanese Patent Laid-Open No. 5-24147 discloses a technical idea of obtaining a coating film with both high-level flexibility and high-level hardness by reacting a polyester resin with a curing agent of a melamine resin which has been modified to have orientability.
However, these prior art techniques have various problems such as those mentioned below.
According to the technique disclosed in Japanese Patent Laid-Open No. 62-236817, it is possible to obtain a coating film with relatively high hardness, but the flexibility of the film at room temperature is unsatisfactory. The technique disclosed in Japanese Patent Laid-Open No. 63-108081 is problematic in that the xylol resistance of the film to be formed from the composition is not good.
The technique disclosed in Japanese Patent Publication No. 56-18032 is to reduce the work strain of the film made from the composition. However, it is difficult to say that, when the composition is used in producing precoated steel sheets, the balance between the elongation and the hardness of the coating film is good.
The technique disclosed in Japanese PCT Application Laid-Open No. 3-500544 is difficult to apply to precoated steel sheets in view of the costs for producing the mesogen-modified polymers, the components that constitute the coating film, and the curing conditions. In addition, this techniques does not include any means of improving the flexibility of the coating film.
The technique disclosed in Japanese Patent Laid-Open Application No. 5-24147 requires complicated steps for producing the modified melamine compounds. At present, any practicable resins cannot be obtained according to this technique.
The techniques disclosed in Japanese Patent Application Laid-Open Nos. 59-11370, 62-195057 and 2-209970 could not satisfy both the high workability and the high hardness that are necessary to the films of precoated steel sheets.
As has been mentioned hereinabove, many of the prior art techniques are to improve the resins themselves that shall be the essential components in coating films, thereby enhancing the hardness and the workability of the films. In general, however, the hardness and the workability of coating films are contradictory to each other. Therefore, it was impossible to obtain coating films having high hardness and high workability, according to the prior art techniques mentioned hereinabove.
Given the situation, accordingly, the object of the present invention is to solve various problems in the prior art such as those mentioned hereinabove, and to provide a coating composition for precoated steel sheets which can form a coating film having both high workability and high hardness and which can be prepared with ease, as well as a precoated steel sheet having a coating film formed from the coating composition.